Laser desorption and imaging of proteins from ice via UV femtosecond laser pulses

We have employed 200-fs, 400-nm laser pulses to desorb intact protein molecular ions directly from a frozen aqueous matrix. The resulting spectra obtained using a variety of proteins varying in molecular weight from 1060 (bradykinin) to 5778 Da (insulin) are compatible with those obtained with traditional matrix-assisted laser desorption/ionization experiments. High-quality spectra could be generated using a fluence of 4.0-9.0 J/cm2 to desorb proteins from an aqueous solution frozen onto metal substrates with a sensitivity in the femtomole range. Although the mechanism behind this effect is still not clear, we speculate that it involves explosive boiling of the ice layer due to rapid heating of the substrate. Imaging experiments conducted on the ice layer suggest that the yield of protein is approximately independent of the film thickness and is very reproducible from shot to shot. The results are particularly significant since they open the possibility of examining a range of biomaterials directly from the in vivo aqueous environment.     

Ultrasmall silver nanopores fabricated by femtosecond laser pulses

Ultrasmall nanopores in silver thin films with a diameter of about 2 nm have been fabricated using femtosecond laser ablation in liquid. Ultrafast laser pulse ablation generates highly nonequilibrium excitated states, from which silver thin films emerge and progressively grow with the assistance of capping agent molecules. During this growth process, capping agent molecules are enclaved within the film, leaving individual ultrasmall pores in the thin film. Our first-principles calculations show that the pore size is critically determined by the dimension of the confined molecules. Our approach advances the capability of optical methods in making nanoscale structures with potential applications in areas such as near-field aperture probes, imaging masks, magnetic plasmonic resonances, and biosensing with individual nanopores.     

Three-dimensional nanostructuring with femtosecond laser pulses

The development of simple laser-based technologies for the fabrication of complicated three-dimensional (3-D) microstructures with a structure size down to 100 nm is reported. These technologies are based on nonlinear multiphoton laser-matter interaction processes allowing to overcome the diffraction limit and to fabricate 3-D structures inside transparent materials. Examples on nanostructuring of metals, dielectrics, and polymers are presented.     

Time integrated detection of femtosecond laser pulses scattered by small droplets

Scattering of femtosecond laser pulses by small droplets has been measured and compared with predictions, yielding some interesting new applications for time integrated detection of the scattered field. The scattering intensity of integrated detection becomes monotonic with droplet size over large regions of scattering angle and morphology dependent resonances are surpressed, opening the way for particle sizing using the scattered intensity. Furthermore, the ripple structure no longer appears in the rainbow region of scattering, simplifying rainbow refractometry significantly. These scattering proporties of femtosecond laser pulses have been demonstrated in the laboratory using a novel Paul trap for levitating single droplets.     

Two-photon fluorescence excitation spectroscopy by pulse shaping ultrabroad-bandwidth femtosecond laser pulses

A fast and automated approach to measuring two-photon fluorescence excitation (TPE) spectra of fluorophores with high resolution (~2 nm) by pulse shaping ultrabroad-bandwidth femtosecond laser pulses is demonstrated. Selective excitation in the range of 675-990 nm was achieved by imposing a series of specially designed phase and amplitude masks on the excitation pulses using a pulse shaper. The method eliminates the need for laser tuning and is, thus, suitable for non-laser-expert use. The TPE spectrum of Fluorescein was compared with independent measurements and the spectra of the pH-sensitive dye 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) in acidic and basic environments were measured for the first time using this approach.     

Optical properties of waveguides fabricated in fused silica by femtosecond laser pulses

With tightly focused femtosecond laser pulses, waveguides are fabricated in fused silica. The guiding and attenuation properties of these waveguides at wavelengths of 514 nm and 1.5 microm are studied. We demonstrate that by changing only the writing speed, waveguides with a controllable mode number can be produced.     

Application of femtosecond laser pulses in biomedical cell technologies

The results are presented of the works in the field of development of equipment, investigation techniques, and technologies for biology and medicine performed in the Joint Institute for High Temperatures of the Russian Academy of Scienses (JIHT RAS). On the base of the new generation infrared femtosecond lasers, the experimental models are developed and manufactured of laser tweezers, scalpel, and the “tweezers-scalpel” combined system. The results are presented of the experimental studies on the noncontact mammalian cell fusion (blastomeres of mouse embryos on day 1.5 of development) by means of the femtosecond laser pulses.     

Noncrystalline micromachining of amorphous alloys using femtosecond laser pulses

Femtosecond laser micromachining of a Zr-based amorphous alloy in air, including measuring the ablation threshold, micro-drilling and trenching, was investigated. The threshold of ablating this amorphous alloy was determined by experiment. Laser-induced ablation and associated damage were examined by means of optical microscope, scanning electron microscopy, transmission electron microscopy and electron diffraction diagram. The results show that conventional processing method induced defects in the vicinity of machined area, such as crystallization, molten trace and spatter, were absent in femtosecond laser ablation area with selected parameters. This indicates that femtosecond laser ablation is a promising method for micromachining amorphous alloys without crystallization.     

Splitting of femtosecond laser pulses by using a Dammann grating and compensation gratings

When a Dammann grating is used to split a beam of femtosecond laser pulses into multiple equal-intensity beams, chromatic dispersion will occur in beams of each order of diffraction and with different scale of angular dispersion because the incident ultrashort pulse contains a broad range of spectral bandwidths. We propose a novel method in which the angular dispersion can be compensated by positioning an m-time-density grating to collimate the mth-order beam that has been split, producing an array of beams that are free of angular dispersion. The increased width of the compensated output pulses and the spectral walk-off effect are discussed. We have verified this approach theoretically and validated it through experiments. It should be highly interesting in practical applications of splitting femtosecond laser pulses for pulse-width measurement, pump-probe measurement, and micromachining at multiple points.     

Raman spectroscopy and luminescent properties of ZnO nanostructures fabricated by femtosecond laser pulses

In this paper, we report the laser-induced periodic structure with different spatial characteristics on the surface of polished ZnO single-crystalline by high repetition rate femtosecond laser pulses. This study demonstrates that, using different laser parameters and irradiation conditions, ZnO nanoripples and nanorods were successfully prepared. We have investigated the surface by means of scanning electron microscope (SEM), Raman scattering and photoluminescence (PL). We propose that second-order harmonic has a strong influence on the formation of nanostructures.     

Distribution of the microcrystallites generated in borate glass irradiated by femtosecond laser pulses

This paper deals with the distribution of generated microcrystallites in borate glass irradiated by 120 fs laser pulses at a central wavelength of 800 nm. Raman spectroscopy is used to investigate the distribution of the high and low temperature phases of barium metaborate crystals generated in the borate glass. In combination with a microexplosion model, bond-breaking induced by laser irradiation is served as the origin of the formation of BBO crystals. Depending on the laser fluence and cooling conditions, the distribution mechanisms have been discussed.     

Time-resolved explosion dynamics of H2O droplets induced by femtosecond laser pulses

Series of time-resolved still images of the explosion dynamics of micrometer-sized water droplets after femtosecond laser-pulse irradiation were obtained for different laser-pulse intensities. Amplified pulses centered around a wavelength of 805 nm with 1-mJ energy and 60-fs duration were focused onto the droplet to initiate the dynamics. Several effects, such as forward and backward plumes, jets, water films, and shock waves, were investigated. Additionally, the influence of different pulse durations produced by chirping the laser pulses was observed.     

Thermal training of functional surfaces fabricated with femtosecond laser pulses

The effect of thermal training of a functional surface fabricated by means of femtosecond laser surface processing on a crystalline silicon (c-Si) surface is found for the first time. Hydrodynamic and thermal properties of the c-Si surface are revealed to be considerably modified by femtosecond laser processing and follow-up thermal training. The carried out experimental studies of wetting, evaporation, and boiling processes on the thermal trained laser processed surface open up ample opportunities in creation of tailored functional surfaces for micro/optoelectronic devices and power engineering applications.     

Micromachining of hardened Portland cement pastes using femtosecond laser pulses

Femtosecond laser pulses (30 fs in length) of various energies were utilised for production of single and multiple overlapping ablation sites on flat polished surfaces of hardened Portland cement pastes. In order to assess the sizes of the ablation sites and possible subsurface laser-induced damage, the ablation sites were investigated using environmental scanning electron microscopy (ESEM) – both from normal top–down view and in cross-sections. Furthermore, approximately 10-μm wide notches were produced using femtosecond pulses on cylindrical microspecimens (150 μm in diameter) of hardened Portland cement pastes. In addition to electron microscopy observations, several microspecimens were investigated using synchrotron-based X-ray computed microtomography (SRμCT). The results suggest that production of “damage-free” samples for micromechanical testing of hardened Portland cements pastes is possible.

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Résumé Des impulsions laser (de durée 30 fs) et d’énergie variable ont été utilisées pour produire une ablation ponctuelle ou linéique à la surface d’un ciment durci de type Portland et préalablement polie. Pour déterminer la taille des impacts et d’éventuels dommages causés par le laser sous la surface, les points d’impact ont été visualisés à l’aide de la technique de l’ESEM (microscope électronique à balayage environnemental). De plus, des piliers de 10 micromètres de diamètre environ ont été réalisés par ablation femtoseconde sur des échantillons cylindriques de 150 microns de diamètre. Pour compléter les observations faites au microscope électronique, certains échantillons ont été observés grace à la technique SRμCT (synchrotron-based X-ray computed microtomography). Les résultats montrent que la production d’échantillons non-endommagés de ciment de type Portland pour des tests micro-mécaniques ultérieurs est possible.

     

Optical damage of the surface of sodium and potassium chlorides by femtosecond laser pulses

Optical damage thresholds for the surface of sodium and potassium chloride crystals irradiated by 80-fs laser pulses have been determined. P-polarized terawatt radiation of a chromium-forsterite laser operating at a wavelength of 1240 nm was incident on a sample surface at an angle of 35°. The optical damage of the crystal surface was observed when the electric field strength reached a critical level of 76 MV/cm for NaCl and 64 MV/cm for KCl.     

Fabrication of beam shapers in the bulk of fused silica by femtosecond laser pulses

We reported a new approach to the fabrication of three-dimensional refractive-index-modified microstructures inside transparent materials by combining two-dimensional writing by scanning the focus of the femtosecond laser pulse and by forming the long filament in the third dimension. In this way, embedded diffractive beam shapers of grid, square, and ring gratings were obtained in the bulk of fused silica by use of a femtosecond laser with a wavelength of 810 nm and a repetition rate of 1 KHz. These structures and their refractive efficiencies were optimized by selection of the appropriate fabrication parameters, including the pulse energy, grating period, scanning speed, and scanning repetition. The good performance of these devices indicates that, owing to its simple and flexible method for fabricating complex phase elements inside transparent materials, this technique has potential applications to integrated optics.     

Generation of high-energy vacuum UV femtosecond pulses by multiple-beam cascaded four-wave mixing in a transparent solid

The generation of ultrashort vacuum UV (VUV) pulses by nondegenerate cascaded four-wave mixing of femtosecond pulses in a thin slide of a large band-gap transparent solid is numerically demonstrated. Using a novel noncollinear multiple-beam configuration, cascaded four-wave mixing of amplified 30 fs Ti:sapphire laser pulses at 800 nm, and their second harmonic in lithium fluoride results in the generation of VUV radiation down to 134 nm with energies in the μJ range and durations comparable to those of the pump pulses. The proposed geometry is advantageous in large dispersion scenarios, namely for generating radiation close to absorption bands. Hence these results set this technique as a promising way to efficiently generate ultrashort VUV radiation in solids for several applications in science and technology.     

Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses

Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply 'heats' the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.     

Temperature distribution in dental tissue after interaction with femtosecond laser pulses

Algebraic and numerical solutions are presented of the temperature rise in dental tissue due to interaction with ultrashort optical radiation. Results of the studies with femtosecond laser pulses show agreement between theory and experiment. A temperature rise of typically 5 K is found for a 40 millisecond train of 7 nJ, 70 fs laser pulses at a repetition rate of 80 MHz. The peak irradiance in our experimental studies was limited to 3x10(6) W/cm(2). Applications include photoacoustic imaging and tomography of dental tissue.